Add E2E coverage for createExecution scheduledAt async and 1-minute boundary rules

### 🔖 Enhancement description `POST /v1/functions/:functionId/executions` already implements validation for scheduled executions, but the boundary cases are not protected by E2E coverage. The current implementation in `src/Appwrite/Platform/Modules/Functions/Http/Executions/Create.php` already enforces these rules: - scheduled executions must run asynchronously - `scheduledAt` must be at least 1 minute in the future - `scheduledAt` more than 1 minute in the future is accepted as a scheduled execution This proposal is not to change the current API behavior. It is to add E2E coverage for the existing contract so these boundaries are less likely to regress. ### 🎤 Pitch This would be a small test-only backend contribution focused on the Functions execution flow. The main value is protecting an existing user-facing API contract with explicit E2E coverage for schedule validation boundaries. The missing cases are: - `async=false` with a valid future `scheduledAt` - `async=true` with `scheduledAt` less than 1 minute ahead - `async=true` with `scheduledAt` more than 1 minute ahead I verified these cases locally in Docker and the targeted test run passed: `docker compose exec appwrite test tests/e2e/Services/Functions/FunctionsCustomServerTest.php --filter=ScheduledExecution` Result: `OK (3 tests, 56 assertions)` I did not find an open issue or PR directly covering these missing E2E cases. If this direction looks good, I can follow up with a small PR limited to Functions execution E2E coverage, without changing production logic. ### 👀 Have you spent some time to check if this issue has been raised before? - [x] I checked and didn't find similar issue ### 🏢 Have you read the Code of Conduct? - [x] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)